The Equilibrium ConstantThe ratio of the rate constants is a constant atThe ratio of the rate constants is a constantatthat temperature, and the expression becomes:Keq=kfk[NO2]2[N O ]=r[N2O4]This is known as theequilibrium constant.

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The Equilibrium Constant•Consider the generalized reaction:bd•The equilibrium expression for this reactionaA + bBcC + dD(from thelaw of mass action) would be:Kc=[C]c[D]d[A]a[B]b[A] [B]The equilibrium constant depends only on thestoichiometry of the reaction, not the mechanism!

Heterogeneous Equilibria•The equilibrium examples we have talked aboutfbdhilib iso far are based on homogeneous equilibria.•Homogeneous equilibriummeans that all ofthe reactants and products are in the samephysical state or phase.•Heterogeneous equilibriumconsiders theequilibrium between components in differentphases (e.g.a solid placed in a liquid).

Heterogeneous Equilibria•The concentrations of pure solids and liquidsare constant.•Solid and liquid concentrations can be obtainedby multiplying the density of the substance byits molar mass.Thttttt tt•These are constants at constant temperature.•For example, doubling the mass of a pure solidwould double its volume and result in anunchanged density (density≈concentration).

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